Difficulty: Intermediate | Prerequisites: Lewis acids/bases, electronegativity, orbital theory (sp3 hybridisation), stereochemistry basics (R/S configuration)
Nucleophilic substitution is one of the most fundamental reaction types in organic chemistry, and SN2 is the first mechanism most courses teach in depth. This chapter introduces alkyl halides as the substrate class, then walks through exactly how and why the SN2 mechanism works the way it does, including the stereochemical outcome (inversion). If you are comfortable with Lewis acid/base concepts and can assign R/S configuration, you are ready for this material. Everything here feeds directly into later chapters on SN1, E1, and E2 reactions, so getting the SN2 logic solid now pays off repeatedly.
An SN2 reaction is a one-step, concerted nucleophilic substitution where a strong nucleophile attacks the back side of an sp3 electrophilic carbon, kicking out a leaving group. The rate depends on both nucleophile and electrophile concentration. The stereochemical result is always inversion of configuration (the "umbrella flip").
Alkyl halide (haloalkane)
An organic compound in which a halogen (F, Cl, Br, I) is bonded to an sp3 carbon. Classified as primary (1°), secondary (2°), or tertiary (3°) based on how many other carbons are attached to the carbon bearing the halogen. Think of it as the backbone substrate for substitution and elimination reactions.
Electrophile
A Lewis acid, an electron-pair acceptor. In SN2, the alkyl halide is the electrophile because the polarised C–X bond gives the carbon a partial positive charge (δ+). In simple terms, it is the species that gets attacked by the nucleophile.
Nucleophile (Nuc)
A Lewis base, an electron-pair donor. It attacks the electrophilic carbon. In simple terms, the nucleophile is the electron-rich species doing the attacking.
Leaving group (LG)
The group that departs with the bonding electrons when the nucleophile attacks. Good leaving groups are weak bases or very stable once they leave (e.g. Cl⁻, Br⁻, I⁻). Think of it as: the easier it is for the group to leave, the faster the reaction goes.
SN2 (substitution, nucleophilic, bimolecular)
A one-step mechanism in which the nucleophile attacks and the leaving group departs simultaneously. "Bimolecular" means both the nucleophile and the electrophile appear in the rate law. In simple terms, everything happens in a single concerted step with no intermediate.
SN1 (substitution, nucleophilic, unimolecular)
A multi-step mechanism where the leaving group departs first to form a carbocation intermediate, then the nucleophile attacks. Requires a weak nucleophile and a substrate that forms a stable carbocation (typically 3°). Covered in contrast here but treated fully later in the course.
Concerted reaction
A reaction in which bond-making and bond-breaking happen at the same time, in one step, with no intermediate.
Backside attack
The nucleophile approaches the electrophilic carbon from the side directly opposite (180°) the leaving group. This geometry is required in SN2.
Inversion of configuration
The stereochemical outcome of SN2: the spatial arrangement of groups around the carbon flips, like an umbrella turning inside out in the wind. If the starting material is R, the product is S (and vice versa).
Transition state (TS)
The highest-energy point along the reaction coordinate. In SN2, the transition state has the carbon in a roughly sp2-like, planar geometry with partial bonds to both the nucleophile and the leaving group.
Rate law
The mathematical expression relating reaction rate to concentrations. For SN2: rate = k[Nuc][Electrophile]. Increasing the concentration of either species increases the rate.
Bond strength of C–X decreases as halogen size increases (C–F is strongest, C–I is weakest)
Bond length of C–X increases as halogen size increases, because orbital overlap becomes poorer with larger atoms
The C–X bond is polarised (carbon is δ+, halogen is δ−), which is what makes the carbon electrophilic
Haloalkanes have higher boiling points than similar-sized alkanes due to dipole-dipole interactions
Types by substitution degree:
Methyl: CH₃X (no other carbons on the C bearing X)
Primary (1°): one carbon attached to the C–X carbon
Secondary (2°): two carbons attached
Tertiary (3°): three carbons attached
Key constraint: SN2 substrates must be sp3-hybridised. Count how many carbons are connected to the C–X carbon to classify.
The general pattern for nucleophilic substitution:
A nucleophile (Lewis base) attacks a substrate/electrophile (Lewis acid)
The leaving group departs as a weak base
The product has the nucleophile bonded where the leaving group used to be
Two mechanistic options exist:
SN2 (concerted): one step, strong nucleophile, good electrophile (methyl or 1°)
SN1 (stepwise): multiple steps, weak nucleophile, substrate that stabilises a carbocation (3°)
When deciding which pathway applies, always ask: (1) What are the reaction components? (2) How do they react? (3) What did you create?
Rate law:
rate = k [Nuc][Electrophile]
Both concentrations matter. Doubling either one doubles the rate. This is what "bimolecular" means.
Mechanism – single concerted step:
The nucleophile donates an electron pair to the electrophilic carbon while the leaving group simultaneously departs with its electron pair. There is one transition state and no intermediate.
Why backside attack, not frontside?
Frontside attack (nucleophile and leaving group on the same side) fails for two reasons:
Electronic repulsion: two species with significant electron density (δ−) crammed into the same region repel each other
Steric hindrance: there is simply not enough room on the same face
A frontside attack would also give retention of configuration, and experimentally that does not happen in SN2 reactions.
Backside attack (180° alignment) works because:
It resolves both the electronic and steric problems by placing the nucleophile as far from the leaving group as possible
The nucleophile attacks the antibonding orbital (σ*) of the C–LG bond, which has its larger lobe on the back side
The transition state has a roughly sp2-like planar carbon with partial bonds to both Nuc and LG
Stereochemical outcome: inversion of configuration
Every SN2 reaction inverts the configuration at the carbon under attack. If the starting material is (R), the product is (S), and vice versa. This is sometimes called the "umbrella flip" because the three remaining groups flip to the other side, like an umbrella inverting in the wind.
All SN2 reactions are concerted, and all SN2 reactions proceed with inversion. No exceptions.
Reaction coordinate diagram:
The diagram shows a single energy hump from reactants to products. The peak is the transition state. If ΔH is negative, products are lower in energy (more stable) than reactants.
Students sometimes think SN2 can proceed via frontside attack. It cannot. The electronic repulsion, steric clash, and orbital alignment all demand backside attack exclusively.
Confusing "inversion" with "the product must be S." Inversion means the configuration flips relative to what it was. R becomes S and S becomes R, but only if the priority rankings do not change when the nucleophile replaces the leaving group. Always reassign R/S on the product.
Assuming SN2 and SN1 are interchangeable. They have different requirements for nucleophile strength, substrate type, and solvent. The mechanism is dictated by conditions.
Forgetting that SN2 requires sp3 carbons only. Vinyl halides and aryl halides do not undergo SN2.
⚠️ You will be asked to draw the SN2 mechanism with correct arrow pushing (Nuc attacks C, electrons flow to LG). Practise until this is automatic.
⚠️ Stereochemistry questions are near-guaranteed. Given a starting material with a chiral centre, you must show inversion. Assign R/S before and after.
⚠️ Rate law questions: know that SN2 is second-order overall (first-order in each reactant).
⚠️ Be able to distinguish SN2 from SN1 based on substrate class, nucleophile strength, and number of mechanistic steps.
True or false: SN2 reactions proceed through a carbocation intermediate.
Fill in the blank: The SN2 rate law is rate = k [][].
True or false: Frontside attack in SN2 leads to retention of configuration.
Fill in the blank: In SN2, the nucleophile attacks the ____ orbital of the C–LG bond.
True or false: A tertiary alkyl halide is an excellent substrate for SN2.
Answers: 1. False (concerted, no intermediate). 2. [Nuc][Electrophile]. 3. False (frontside attack does not occur in SN2; backside attack gives inversion). 4. Antibonding (σ*). 5. False (too much steric hindrance).
Q: Draw the product of the following SN2 reaction: (S)-2-bromobutane + NaCN in THF. Include stereochemistry.
A: The CN⁻ nucleophile attacks the carbon bearing the bromine from the back side. Bromine leaves. The configuration inverts from (S) to (R). The product is (R)-2-cyanobutane (reassign priorities after substitution to confirm the R/S label).
Q: Why does an SN2 reaction not proceed through a frontside attack?
A: Frontside attack places the nucleophile and the leaving group on the same face of the carbon. This creates electronic repulsion between two electron-rich species and steric crowding. Additionally, the antibonding orbital that the nucleophile needs to access has its major lobe on the opposite side. The result would be retention of configuration, which is not observed experimentally.
Q: Write the rate law for an SN2 reaction between CH₃Br and NaOH. What happens to the rate if you double the concentration of NaOH only?
A: Rate = k[OH⁻][CH₃Br]. Doubling [OH⁻] doubles the rate, because the reaction is first-order in nucleophile.
Q: Explain why SN2 is described as "bimolecular."
A: Both the nucleophile and the substrate (electrophile) are involved in the single rate-determining step. Both appear in the rate law, making it second-order overall (bimolecular).
SN2 contrasts directly with SN1, which you will study alongside E1 and E2 elimination. Understanding why SN2 requires a strong nucleophile and an unhindered substrate makes it much easier to sort reactions into the correct mechanism later.
Stereochemistry from earlier chapters (R/S assignment, chirality) is applied here. If R/S assignment is shaky, revisit it now, because inversion questions assume fluency.
The Lewis acid/base framework introduced early in the course is exactly what drives nucleophilic substitution: the nucleophile is the Lewis base, the substrate is the Lewis acid.
SN2, SN2 mechanism, nucleophilic substitution bimolecular, backside attack, inversion of configuration, umbrella flip, Walden inversion, alkyl halide, haloalkane, leaving group, nucleophile, electrophile, rate law second order, concerted mechanism, transition state, antibonding orbital, organic chemistry chapter 6